1. Field of the Invention
The present invention relates generally to mobile communication systems, and more particularly, to a device and method for minimizing the power consumption in a mobile communication terminal by controlling the bias voltage/current of a radio frequency power amplifier.
2. Description of the Related Art
In a mobile communication terminal, the linear characteristic and power consumption represent the primary focus of concern in the design of a power amplifier. That is, the power amplifier is designed in consideration of the power efficiency and linearity at the maximum RF (Radio Frequency) output. A class AB power amplifier is generally used to satisfy the dual requirements of maximum power efficiency and linearity. A corresponding impedance matching and bias state for each amplification stage is then determined. However, the mobile communication terminal is not always driven at the maximum output, and its transmitting output may vary according to a distance from a base station and its velocity. For distances closer to the base station, the transmitting power of the mobile communication terminal is reduced. By contrast, if the mobile communication terminal is further from the base station while still being situated within a radius of a cell or its velocity increases, an RF output approximating the maximum output of the amplifier is transmitted.
FIG. 1 is a block diagram illustrating a conventional power amplifier to which a fixed bias voltage and current are supplied to obtain the maximum output of a mobile communication terminal.
Referring to FIG. 1, amplifiers 101 and 102 amplify an input RF signal according to an input bias voltage and bias current each having a fixed value. A power supply 103 supplies the fixed bias voltage to the amplifiers 101 and 102. A current controller 104 supplies the fixed bias current to the amplifiers 101 and 102.
The power amplifier constructed in the manner described above operates by a fixed bias voltage and bias current. However, such a power amplifier exhibits a remarkably reduced power added efficiency (P.A.E., that is, the ratio of an AC output power minus an AC input power to a DC power of the power amplifier) at its low power output level, as indicated by the graph of FIG.6. The graph illustrates P.A.E. where the bias voltage and bias current are fixed at prescribed values to obtain the maximum RF output. The graph illustrates that unnecessary DC power is consumed because the class AB power amplifier which normally operates around the maximum output level operates in class A mode as the output level decreases. One proposed method for solving this problem is to selectively drive a power amplifiers connected either in series or in parallel. parallel.
FIG. 2 is a block diagram illustrating another conventional power amplifier in which amplifiers are connected in parallel. In FIG. 2, amplifiers connected in parallel are switched according to a desired output level (i.e., high or low).
Referring to FIG. 2, an RSSI(Received Signal Strength Indication) detector 206 detects a strength of a signal received from a base station (not shown) and informs a controller 205 of the detected signal strength. The controller 205 controls switches 201 and 207 to activate a corresponding amplifier according to the output of the RSSI detector 206 and further controls a power supply 208 to supply power to the activated amplifier. The switches 201 and 207 switch an input RF signal to amplifiers 202 and 203 or amplifier 204 under the control of the controller 205. The power supply 208 supplies a fixed voltage to either the amplifier 202 and 203 or the amplifier 204. The non-selected amplifiers(i.e., 202, 203 or 204) will have its voltage cut off. The amplifiers 202, 203 and 204 are used to pair amplify the input RF signal.
In operation, the RSSI detector 206 detects the strength of a received signal. If the detected signal strength is greater than a prescribed level, the controller 205 controls the switches 201 and 207 to amplify the input RF signal through the amplifier 204 having a low output level and to output the amplified signal to an output RF terminal RFout, and simultaneously sets a variable power voltage Va1 to 0 volts. If the detected signal strength is less than a prescribed level, the controller 205 controls the switches 201 and 207 to amplify the input RF signal through the amplifiers 202 and 203 having a high output level and to output the amplified signal to the output RF terminal RFout, and simultaneously sets a variable power voltage Va2 to 0 volts. Namely, for a low output level, the amplifier which dissipates a low DC power is used to suppress unnecessary power dissipation by cutting off a power supplied to the amplifier necessary for a high DC power.
A switching method for serially connected amplifiers is similar to the parallel switching method described and will therefore not be described in detail herein. In brief, DC power consumption can be reduced for a low output level by cutting off a bias voltage supplied to an output terminal amplifier in serially connected amplifiers. Here, an RF signal is amplified at a front terminal, switched at the output terminal amplifier and bypassed to an output RF terminal RFout.
The above switching methods do not provide a complete solution as they suffer from the following problems. First, cost rises and volume increases due to an additional amplifier connected in parallel. Furthermore, it is difficult to optimize the efficiency of the amplifier due to the fixed bias voltage and current. Second, a circuit becomes complicated whenever output levels exceed two. While prior art solutions can increase efficiency to within a limited range, optimizing the efficiency remains difficult.
It is, therefore, an object of the present invention to provide an apparatus and method for minimizing the power consumption of a mobile communication terminal by controlling a bias current and bias voltage according to a desired transmitting output power level.
To achieve the above object of the present invention, a device for controlling an output power level in a mobile communication terminal for transmitting an RF signal amplified by a power amplifier through an antenna and receiving a signal from a base station through the antenna, includes an RSSI detector for detecting a strength of a signal received from the base station, and a controller having a memory for storing bias voltage control values and bias current control values corresponding to strengths of received signals, wherein the controller reads the corresponding bias voltage control value and bias current control value in response to the detected signal strength and supplies the read values to the power amplifier as control signals.
In another aspect of the present invention, there is provided a method for controlling an output power level in a mobile communication terminal having a memory for storing bias voltage control values and bias current control values corresponding to strengths of received signals, a power amplifier for amplifying an RF signal, and an antenna for transmitting the RF signal power-amplified by the power amplifier and receiving a signal from a base station. The method includes the steps of detecting a strength of a signal received from the base station, reading the corresponding bias voltage control value and bias current control value from the memory in response to the detected signal strength, and supplying the read bias voltage control value and bias current control value to the power amplifier.